1. Field of the Invention
The present invention relates to the field of electrochemical devices, and more specifically solid oxide fuel cells, SOFC, and oxygen generators.
2. Background
Steadily increasing demand for power and the atmospheric build up of greenhouse and other combustion gases has spurred the development of alternative energy sources for the production of electricity. Fuel cells hold the promise of an efficient, low pollution technology for generating electricity. Because there is no combustion of fuel involved in the process, fuel cells do not create any of the pollutants that are commonly produced in the conventional generation of electricity by boilers or furnaces and steam driven turbines.
The present cost of electrical energy production from fuel cells is several times higher than the cost of the same electrical production from fossil fuels. The high cost of capitalization and operation per kilowatt of electricity produced has delayed the commercial introduction of fuel cell generation systems.
Solid oxide fuel cells offer the potential of high volumetric power density combined with fuel flexibility. Considerable progress is being made in raising the performance of solid oxide fuel cells, and as an example, one of the present inventors was the first to demonstrate that power densities of as much as 2 W/cm2 could be obtained for supported thin-film yttria-stabilized zirconia (YSZ) solid oxide fuel cells, at 800° C., see S. de Souza, S. J. Visco, and L. C. De Jonghe, “Reduced-temperature solid oxide fuel cell based on YSZ thin-film electrolyte,” J. Electrochem. Soc., 144, L35-L37 (1997), the contents of which are hereby incorporated in their entirety for all purposes. While this result was encouraging, further reductions in temperature are necessary. Such reduction in operating temperature on the one hand makes the use of metallic interconnects and support electrodes possible, allowing for vast cost reduction, but on the other hand requires new ways of configuring fuel cells so that current can be collected with minimal resistive loss.
A conventional fuel cell is an electrochemical device that converts chemical energy from a chemical reaction with the fuel directly into electrical energy. Electricity is generated in a fuel cell through the electrochemical reaction that occurs between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically oxygen in air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase of the fuel or oxygen. Water, heat and electricity are the only products of one type of fuel cell system designed to use hydrogen gas as fuel. Other types of fuel cells known in the prior art include molten carbonate fuel cells, phosphoric acid fuel cells, alkaline fuel cells, and proton exchange membrane fuel cells. Because fuel cells rely on electrochemical rather than thermo-mechanical processes in the conversion of fuel into electricity, the fuel cell is not limited by the Carnot efficiency experienced by conventional mechanical generators.
FIG. 1 illustrates a cross section of a fuel cell, in particular a solid oxide fuel cell (SOFC) (10). The SOFC unit consists of two electrodes, an anode (16) and a cathode (18) separated by an electrolyte (17). In this example, a Nickel-yttria-stabilized zirconia cermet (Ni/YSZ) is the material used for the anode (16). Lanthanum strontium maganite (LSM) is the material used for the cathode (18) and yttria-stabilized zirconia (YSZ) is used for the electrolyte. Many other combinations of materials may be used to construct a SOFC. Fuel (11), such as H2 or CH4 (the present invention may be used with fuels other than H2 and CH4) is supplied to the anode (16), where it is oxidized by oxygen ions (O2−) from the electrolyte (17), which releases electrons to the external circuit. On the cathode (18) an oxidant such as O2 or air is fed to the cathode, where it supplies the oxygen ions from the electrolyte by accepting electrons from the external circuit. The electrolyte (17) conducts these ions between the electrodes, maintaining overall electrical charge balance. The flow of electrons in the external circuit provides power (15), which may be siphoned off from the external circuit for other uses. Reaction products (12) are exhausted off the device. Excess air (14) may be passed through the device.
FIG. 2 illustrates a basic planar design for a solid state electrochemical device, for example a solid oxide fuel cell (SOFC). Typically and multitude of cells are “stacked” to make a “stack”. In reality, there is no space between the stacks as shown in FIG. 2. The cell (10) includes an anode 16 (the “fuel (fuel 11) electrode”) and a cathode (18) (the “air, (oxidant 13) electrode”) and a solid electrolyte (17) separating the two electrodes. In conventional SOFCs, the electrolytes are typically formed from ceramic materials, since ceramics are able to withstand the high temperatures at which the devices are operated. For example, SOFCs are conventionally operated at about 950° C. This operating temperature is determined by a number of factors, in particular, the temperature required for the reformation of methane to produce hydrogen and reaction efficiency considerations. Also, typical solid state ionic devices such as SOFCs have structural element on to which the SOFC is built. In conventional planar SOFCs the structural element is a thick solid electrolyte plate such as yttria stabilized zirconia (YSZ); the porous electrodes are then screen printed onto the electrolyte.
In the case of a typical solid oxide fuel cell, the anode is exposed to fuel and the cathode is exposed to an oxidant in separate closed systems to avoid any mixing of the fuel and oxidants due to the exothermic reactions that can take place with hydrogen fuel.
The electrolyte membrane is normally composed of a ceramic oxygen ion conductor in solid oxide fuel cell applications. In other implementations, such as gas separation devices, the solid membrane may be composed of a mixed ionic electronic conducting material (“MIEC”). The porous anode may be a layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (“cermet”) that is in contact with the electrolyte membrane on the fuel side of the cell. The porous cathode is typically a layer of a mixed ionically and electronically-conductive (MIEC) metal oxide or a mixture of an electronically conductive metal oxide (or MIEC metal oxide) and an ionically conductive metal oxide.
Solid oxide fuel cells normally operate at temperatures between about 900° C. and about 1000° C. to maximize the ionic conductivity of the electrolyte membrane. At appropriate temperatures the oxygen ions easily migrate through the crystal lattice of the electrolyte. However, most metals are not stable at the high operating temperatures and oxidizing environment of conventional fuel cells and become converted to brittle metal oxides. Accordingly, solid-state electrochemical devices have conventionally been constructed of heat-tolerant ceramic materials. However, these materials tend to be expensive and still have a limited life in high temperature and high oxidation conditions. In addition, the materials used must have certain chemical, thermal and physical characteristics to avoid delamination due to thermal stresses, fuel or oxidant infiltration across the electrolyte and similar problems during the production and operation of the cells.
Since each fuel cell generates a relatively small voltage, several fuel cells may be associated to increase the capacity of the system. Such arrays or stacks generally have a tubular or planar design. Planar designs typically have a planar anode-electrolyte-cathode deposited on a conductive interconnect and stacked in series. However, planar designs are generally recognized as having significant safety and reliability concerns due to the complexity of sealing of the units and manifolding a planar stack.
In addition, conventional stacks of planar fuel cells operated at the higher temperature of approximately 1000° C. have relatively thick electrolyte layers compared to the porous anode and cathode layers applied to either side of the electrolyte and provides structural support to the cell. However, in order to reduce the operating temperature to less than 800° C., the thickness of the electrolyte layer has been reduced from more than 50-500 microns to approximately 5-50 microns. The thin electrolyte layer in this configuration is not a load bearing layer. Rather, the relatively weak porous anode and cathode layers must bear the load for the cell. Stacks of planar fuel cells supported by weak anodes or cathodes may be prone to collapse under the load.
Prior art stacks suffer from the fact that all four sides (if rectangular) are coupled to each other. This arrangement induces thermal and mechanical stresses during operation that cause various failures within cells in the stack, decreasing performance and lifetime of the device. In one attempt to solve the problems of the prior art U.S. Published application no. 20030096147 A1, published May 22, 2003, the contents of which are hereby incorporated by reference in its entirety for all purposes, discloses solid oxide fuel cell assemblys having packet elements having an enclosed interior formed in part by one or more compliant solid oxide sheet sections with a plurality of anodes disposed within the enclosed interior.